1. Field of the Invention
The present invention relates to a DC/DC conversion apparatus that includes a LLC full-bridge circuit.
2. Description of the Related Art
In the prior art, a switch power supply is one power supply that utilizes a modern power electronic technology to control a ratio of a turn-on time and a turn-off time of a switch and maintain a stable output voltage, in which a DC/DC conversion apparatus, i.e., direct current-direct current conversion circuit, is a voltage transformer that effectively converts a DC input voltage into a fixed DC output voltage. Generally, the DC/DC conversion apparatus is divided into three types: a boost DC/DC transformer, a buck DC/DC transformer, and a boost-buck DC/DC transformer, and three types of control may be utilized according to the requirements. Specifically, by utilizing energy storage characteristics of a capacitor and an inductor, high-frequency switching actions is performed by a controllable switch (MOSFET, etc.), inputted electric energy is stored in the capacitor or the inductor, and the electric energy is released to a load so as to provide energy when the switch is turned off. The ability to output power or a voltage is related to a duty cycle, i.e., a ratio of a turn-on time of the switch and the whole cycle of the switch.
However, as the power electronic technology is developing rapidly, requirements, such as more high-frequency operation, high conversion efficiency, high power density, low noise, and other requirements have been proposed to a switch power supply.
FIG. 6 shows an existing DC/DC conversion apparatus 100 that includes a LLC full-bridge circuit. As shown in FIG. 6, the DC/DC conversion apparatus 100 includes a direct-current (DC) voltage source V10, four switch elements Q1˜Q4, an oscillation circuit 20 including an inductor Lr and a capacitor Cr, and a transformation circuit 40 including a transformer 30 and a rectification circuit. In the DC/DC conversion apparatus 100, turn-on and turn-off of individual switch elements Q1˜Q4 are controlled, so as to control energy to be transmitted from a primary side Tr1 of the transformer 30 to its secondary side Tr2.
CN patent CN201110394250.4 provides a DC/DC converter, power transformer, and control methods thereof. It has been broadly mentioned in CN201110394250.4 that asymmetry of a ratio of times in two directions of a power supply voltage being loaded on a LLC resonance loop is changed, so as to change a gain. However, the gain varies due to many factors, such as a variation of the load in a curve that shows the above asymmetry varies, which is not a static period. Parameters involved in changing the gain in the above manner are numerous, and control of the parameters is quite complicated.
For the individual switch elements Q1˜Q4 in the DC/DC conversion apparatus 100 as shown in FIG. 6, their control sequences are shown in FIG. 7.
As shown in FIG. 7, a duty cycle of each switch element Q1˜Q4 is 50%. At time t0, the switch elements Q1 and Q4 are turned on, the switch elements Q2 and Q3 are turned off, and a voltage Vc+− applied on the oscillation circuit 20 including the inductor Lr and the capacitor Cr is a positive value. At this moment, a current ILLC flowing through the oscillation circuit 20 has a positive value and increases gradually. Then, at time t1, the switch elements Q2 and Q3 are turned on and the switch elements Q1 and Q4 are turned off. At this moment, since the voltage is varying intermittently, the voltage Vc+− applied on the oscillation circuit 20 instantly becomes a negative value. However, since the current is varying consecutively, as shown in FIG. 7, at time t1, when the voltage Vc+− applied on the oscillation circuit 20 instantly becomes a negative value, the current ILLC flowing through the oscillation circuit 20 is still a positive value although it decreases gradually. In other words, from time t1 (i.e., a time of switching the switches) to the time of the current ILLC flowing through the oscillation circuit 20 being reduced to zero, the current ILLC flowing through the oscillation circuit 20 has a opposite direction from the voltage Vc+− applied on two terminals of the oscillation circuit 20. The result is that, since energy to be outputted to the secondary side Tr2 of the transformer 30 is a product of the voltage Vc+− and the current ILLC, as shown in FIG. 7, the energy to be outputted to the secondary side of the transformer 30 is negative (i.e., the energy flows reversely from the oscillation circuit 20 to the DC voltage source V10) within a time period of A→B, and the energy will oscillate between the DC voltage source V10 and the oscillation circuit 20 after the time period of A→B. The oscillation between the DC voltage source V1 and the oscillation circuit 20 and a resistance present on a current path of the oscillation circuit 20 will result in an unnecessary loss.
Likewise, at time t2, the switch elements Q1 and Q4 are turned on and the switch elements Q2 and Q3 are turned off. At this moment, since the voltage is varying intermittently, the voltage Vc+− applied on the oscillation circuit 20 instantly becomes a positive value. However, since the current is varying consecutively, as shown in FIG. 7, at time t2, when the voltage Vc+− applied on the oscillation circuit 20 instantly becomes a positive value, the current ILLC flowing through the oscillation circuit 20 is still a negative value although it increases gradually. The result is that, as shown in FIG. 7, the energy to be outputted to the secondary side Tr2 of the transformer 30 is negative (i.e., the energy flows reversely from the oscillation circuit 20 to the DC voltage source V10) and oscillates between the DC voltage source V10 and the oscillation circuit 20 within a time period of C→D. A resistance present on the current path of the oscillation circuit 20 will result in an unnecessary loss.
In addition, a gain perspective should also be considered. Assume that a gain of the DC/DC conversion apparatus 100 is 1, switching frequencies of the individual switch elements Q1˜Q4 in the DC/DC conversion apparatus 100 are equal to a resonance frequency of the oscillation circuit 20. At this moment, a loss will not be generated in the DC/DC conversion apparatus 100. However, if the gain is less than 1, an input voltage Vin is certainly greater than an output voltage Vout. Since the duty cycles of the switch elements Q1˜Q4 are 50%, ILLC=Iout (i.e., the current ILLC flowing through the oscillation circuit 20 is equal to an output current) and input energy (i.e., a product of Vin and ILLC) is certainly greater than output energy (i.e., a product of Vout and Iout). Wherein this extra portion (i.e., a value of Vin*ILLC−Vout*Iout) has been consumed in the DC/DC conversion apparatus 100.
In other words, in the existing DC/DC conversion apparatus 100 as shown in FIG. 6, turn-on and turn-off of the individual switch elements Q1˜Q4 are controlled by the duty cycle 50% such that a portion of the energy will be consumed at the primary Tr1 of the transformer 30, which results in a reduced output power such that the gain of the DC/DC conversion apparatus 100 also decreases.
On the other hand, in the DC/DC conversion apparatus that employs a LLC full-bridge circuit, there is also a problem of a loss of a switch (i.e., MOSFET, etc.). For the problem of the switching loss, a soft-switching technology is usually utilized in the present technical field.
Soft-switching is in contrast to hard-switching. Generally, resonance is introduced before and after the process of the turn-on and the turn-off, such that a voltage of the switch before it is turned on is firstly reduced to zero and a current of the switch before it is turned off is firstly reduced to zero, which can eliminate an overlap of the voltage and the current of the switch during the turn-on and the turn-off and decrease their variation ratio so as to greatly reduce or even eliminate the switching loss. At the same time, a variation ratio of a voltage and a current of the switch during the turn-on and the turn-off is restricted by the resonance process, which also significantly decreases the noise of the switch.
For the process of turning off the switch, an ideal soft turn-off process is such that the current is firstly reduced to zero and then the voltage increases slowly to an off-state value. At this moment, a turn-off loss of the switch is approximately zero. Since the current of the device before it is turned off has been reduced to zero, the problem of inductive turn-off has been solved. This is usually referred to as a zero current switch (ZCS). In addition, for the process of turning on the switch, an ideal soft turn-on process is such that the voltage is firstly reduced to zero and then the current increases slowly to an on-state value. At this moment, turn-on loss of the switch is approximately zero. Since the voltage of a junction capacitance of the device is also zero, the problem of capacitive turn-on has been solved. This is usually referred to as a zero voltage switch (ZVS).
In the prior art, in order to decrease the loss of the switch when it is turned on or even achieve the zero current switch (ZCS) and/or the zero voltage switch (ZVS), sequences of turning on and turning off the individual switch elements Q1˜Q4 have to be adjusted appropriately.
In the DC/DC conversion apparatus 100 as shown in FIG. 6, when the individual switch elements Q1˜Q4 are controlled by control sequences as shown in FIG. 7, the switch elements Q2 and Q3 are turned on at time t1. At this moment, since the DC power supply voltage Vin of the DC voltage source V10 is directly applied at two terminals of the switch elements Q3 and Q2, for the switch elements Q2 and Q3, it is difficult to achieve the zero voltage switch (ZVS), and vice versa. At time t2, the switch elements Q1 and Q4 are turned on. At this moment, since the DC power supply voltage Vin of the DC voltage source V10 is directly applied at two terminals of the switch elements Q1 and Q4, for the switch elements Q1 and Q4, it is also difficult to achieve the zero voltage switch (ZVS).
FIG. 8 shows a relationship between a switching frequency (fs, horizontal axis) and a gain (vertical axis). When the output voltage is constant but the load becomes smaller, its output current will become smaller and, thus, the gain will decrease. When the switching frequency is greater than a resonance frequency, the gain may decrease by controlling the switching frequency to increase.
However, in the DC/DC conversion apparatus 100 as shown in FIG. 6, when the load becomes lighter, increasing the switching frequency of the switch will result in an increased switching loss.